In order for it to be possible to handle the load on a truck effectively, load-handling equipment is necessary. The most common examples of such equipment are tipping gear and cranes. Also common are hook loaders, refuse-handling units, rotating cement mixers, flushing units and air compressors for loading or unloading bulk loads.
In order to utilize the driving power of the vehicle engine to drive the load-handling equipment as well, a power take-off is required. The driving power from the power take-off can be transmitted either mechanically via a propeller shaft or belts or hydraulically by virtue of a hydraulic pump being mounted on the power take-off.
Power take-offs are divided into clutch-independent and clutch-dependent power take-offs. The clutch-dependent power take-offs are mounted on the gearbox and are usually driven by the intermediate shaft of the gearbox. This means that the power take-off is clutch-dependent; that is to say, the power take-off stops when the disk clutch between the engine and gearbox of the vehicle is disengaged. Depending on whether or not the gearbox is equipped with a split gear, the ratio between the engine and the power take-off can be influenced.
Automatic gearboxes of the automated stage-geared gearbox type have become increasingly common in heavy-duty vehicles as microcomputer technology has developed further and made it possible, with a control computer and a number of actuators, for example servo motors, to precision-regulate engine speed, engagement and disengagement of an automated disk clutch between the engine and the gearbox and also the internal coupling means of the gearbox in such a way and in relation to one another so that gentle shifting is always obtained at the correct rotational speed.
The advantage of this type of automatic gearbox compared with a conventional automatic gearbox constructed with planetary gear stages and with a hydrodynamic torque converter on the input side is on the one hand that, especially as far as use in heavy-duty vehicles is concerned, it is simpler and more robust and can be manufactured at a considerably lower cost than the conventional automatic gearbox and on the other hand that it has higher efficiency, which means lower fuel consumption is possible.
When shifting of any unsynchronized gears included in the gearbox takes place, the speed is synchronized by precision control of the rotational speed of the engine. The disk clutch between the engine and the gearbox is therefore engaged so that the rotational speed of the main shaft and the rotational speed of the intermediate shaft are adapted to a new ratio selected. The engine speed is regulated depending on the fuel quantity injected and the engine brake.
When the shifting of synchronized gears takes place, the speed adaptation is effected by means of synchronizing rings.
According to known arrangements for the abovementioned type of automated stage-geared gearbox, clutch-dependent power take-offs are suitable for load-handling equipment which is used when the vehicle is stationary or when the vehicle is driven only in the starting gear, for example tipping units, cranes, hook loaders, pumps for emptying/filling from various containers and air compressors for loading or unloading bulk loads. As the power take-off loads the synchronization system when shifting takes place, it is not permitted for the power take-off to be engaged when shifts take place when the vehicle is being driven. The clutch-dependent power take-off is engaged when the engine of the vehicle is running at idling speed, the gearbox of the vehicle is in its neutral position (or alternatively a starting gear) and the disk clutch between the engine and the gearbox is disengaged. When the disk clutch is engaged, the engine drives the power take-off.
Typically, the vehicle is stationary during this engagement procedure. This also applies for disengagement of the power take-off.
The driver of a vehicle with a clutch-dependent power take-off of known design is therefore limited to using the clutch-dependent power take-off when the vehicle is stationary or if a starting gear is engaged owing to the fact that the vehicle can only be driven in this gear at the same time as the power take-off is engaged. When the vehicle is in motion, the power take-off is therefore restricted to being utilized only in a single starting gear in the selected driving direction. Shifting when the vehicle is in motion with clutch-dependent power take-off engaged is not possible according to these known designs. Here, motion means that the speed of the vehicle is high enough that the lowest gear of the vehicle can be engaged without there being a risk of the engine being throttled down.
A need therefore exists in a vehicle equipped with a clutch-dependent power take-off and an automated stage-geared gearbox for it to be possible to use the clutch-dependent power take-off of the vehicle when the vehicle is in motion and with the possibility of shifting between all the gears of the vehicle throughout the speed range of the vehicle.